1. Field of the Invention
The present invention relates to a reflective or transflective liquid crystal display (LCD) provided with a reflective plate having a surface with bumps, and a fabrication method thereof.
2. Description of the Related Art
In general, liquid crystal displays are divided into transmissive LCDs and reflective LCDs according to the type of the light source. The transmissive LCDs use a backlight as a light source while the reflective LCDs do not use a backlight, but use external light as the light source.
The transmissive LCD using the backlight as the light source displays a bright image even in a dark environment but has a disadvantage of high power consumption. On the other hand because the reflective LCD obtains light from external natural light or an artificial light, it has an advantage of a low power consumption compared with the transmissive LCD.
This advantage of the reflective LCD leads to the desirability of the reflective LCD. The reflective LCD, however, has a disadvantage in that it is difficult to use in a dark environment. To overcome this disadvantage, there is a need for a transflective LCD that can be used as both a reflective LCD and a transmissive LCD.
FIG. 1A is a sectional view of a reflective LCD according to the related art and FIG. 1B is a graph showing a reflection angle characteristic of the reflective LCD shown in FIG. 1. An LCD according to the related art includes a lower substrate 110, an upper substrate 100 and a liquid crystal layer 120 interposed therebetween. The lower substrate 110 includes a lower base substrate 111, a gate electrode 112, a gate insulating layer 115, a semiconductor layer 116, an ohmic contact layer 117, a thin film transistor (TFT) including source and drain electrodes 113 and 114, an organic insulating layer 118 formed on the lower base substrate including the TFT, a reflective plate 119 formed on the organic insulating layer 118, and a lower alignment film (not shown) formed on the organic insulating layer 118 including the reflective plate 119.
The upper substrate 100 includes an upper base substrate 101, a black matrix 102 formed on an inner surface of the lower base substrate 101 corresponding to the TFT, a color filter 103 formed on both sides of the black matrix 102, a common electrode 104 formed on the black matrix 102 and the color filter 103, and an upper alignment film (not shown) formed on the common electrode 104. If the reflective plate 119 is formed of an opaque metal, it functions to reflect external light as well as a pixel electrode. On the contrary, if the reflective plate 119 is formed of ITO, i.e., a transparent electrode, it only functions as a pixel electrode to reflect external light without a function.
While FIG. 1B shows an example of how the reflective plate 119 reflects external light, it is also possible to form a separate pixel electrode.
However, the conventional reflective LCD as described above has a drawback in that its viewing angle characteristic is not good. If external light is incident into the upper substrate having a flat mirror type reflective plate with an incident angle of ‘I’, the external light is reflected with a reflection angle ‘R’ with respect to a normal line of the upper substrate via the reflective plate and the liquid crystal layer according to Fermat's principle. At this time, the reflecting light has the reflection angle ‘R’ that is the same magnitude as the incident angle ‘I’, but is opposite in sign and direction to the incident light.
From FIG. 1B shows the reflection angle characteristic of the conventional reflective LCD. The horizontal axis represents a reflection angle of external light and the vertical axis represents the intensity of the reflected light at each reflection angle. In a general reflective LCD, an incident angle of external light for example is −30°, and is reflected at a reflection angle 150 of 30°.
In the aforementioned conventional reflective LCD external light is reflected and concentrated toward the reflection angle 150. Thus, because the reflecting light is nearly reflected toward a front reflection angle range 160 of 0–20° which corresponds to a typical user viewing locations the reflective LCD fails to perform its role as a display.
Accordingly, there remains a need for a technique for reflecting external light with the front reflection angle ranging from 0 to 20° that is the typical user viewing location.
In order to overcome the problems with viewing angle in conventional reflective LCD's technique of forming a scattering particle layer in the upper substrate or in the color filter layer has been proposed. This techniques improves the viewing angle, but does not obtain a satisfactory viewing angle.
Another proposed technique, uses a reflective plate having a bump structure. This technique provides a reflective plate that is not a flat mirror type but has an embossed surface. By doing so, the reflected light is spead over a range of viewing angles.
Until now, the technique using the reflective plate having the bump structure has been widely used to improve the poor viewing angle of the conventional reflective LCD. Much research relating to methods for forming such a bump structure is under way.
FIG. 2A is a sectional view of a reflective LCD employing a reflective plate having a bump structure according to the related art and FIG. 2B is a graph showing a reflection angle characteristic of the reflective LCD shown in FIG. 2A.
The reflective LCD employing a reflective plate 200 having a plurality of bumps 270 shown in FIG. 2A has a similar structure to that of FIG. 1 but has a difference in the shape of the reflective plates. In other words, the surface of the reflective plate 200 is not flat like a mirror has a regular configuration or a random configuration of bumps. Due to the existence of the bumps 270, external light that is incident with an incident angle ‘I’ is not reflected with a fixed reflection angle ‘R’ that is the same as the incident angle ‘I’.
The graph of FIG. 2B shows the reflection angle characteristic of the reflective LCD employing the reflective plate 200 having the bumps 270. Like FIG. 1B shows the horizontal axis of FIG. 2B represents a reflection angle of the reflected light and the vertical axis represents the intensity of the reflected light.
A comparison of the graph of FIG. 2B with that of FIG. 1B shows that the reflection angle range of the reflected external light is widened. The reflected light corresponding to a reflection angle 230 is referred to as a ‘reflection component’ 250, and the reflected light that is widely distributed other than the reflection angle 230 is referred to as ‘haze component’ 260. The reflection component 250 has the greatest intensity. When the incident angle of the external light is −30°, the incident light is not just reflected at the reflection angle 230, but the reflecting light is also reflected toward the front reflection angles 240 ranging from 0–20° corresponding to the typical user location in front of the display.
FIG. 3 is a schematic view showing scattering and reflection of external light in the reflective plate shown in FIG. 2A. Assuming that the refraction index in air is n1, and the value of n1 is 1. And, assuming that the refraction index of the liquid crystal layer 310, through which the external light passes the value of n2 is approximately 1.5.
According to Snell's law, when an incident angle of external light is 30°, a refracted angle is expressed by an equation of sin−1(n1/n2*sin 30°). Hence, when n1=1, n2=1.5 in the above equation, the refractive angle is approximately 20°.
The light that is incident on the bumped surface 320 of the reflective plate 300 is reflected with a reflection angle with respect to the normal of the substrate according to Fermat's principle. The reflected light has the reflection angle that is the same in magnitude as the incident angle, but is opposite in its sign and direction to the incident light. Then, because the light that is incident onto the bumped surface 320 has different normal at different points of the bumped surface 320 and accordingly the reflection angle is not fixed at 20° but spreads over a range of reflection angles.
Accordingly, external light that an initial incident angle of 30° is diffused even towards the reflection angles ranging from 0–20° corresponding to the typical user viewing location and is reflected to improve the low luminance characteristic that is a disadvantage of the conventional mirror type reflective plate in the typical user viewing location.
However, by simply forming bumps in the reflective plate, it is difficult to achieve the uniformity that allows external light to be uniformly reflected toward the front reflection angle corresponding to the typical user viewing location. Also, there is a drawback in that the intensity of the light reflected toward the front reflection angle corresponding to the typical user viewing location is not sufficiently strong enough for satisfactory user viewing.